In Griffith's Introduction to electrodynamics, he uses a cursive r as the distance between a charge and a test point. most of the time to find this distance we subtract the two position vectors and find the magnitude, but occasionally we use the law of cosines. Now yes I know we use the law of cosines when the angle is not 90 degrees in statics problems. So I assumed that was the indicator to when to use the law of cosines. However immediately after assuming this, we just subtracted a second set of vectors and found the magnitude of two vectors that didnt make a right triangle. Can someone just very clearly and slowly tell me when to use the law of cosines, and when i can just subtract two vectors and find the magnitude?